[Fix #45] Make Rails/Delegate aware of self

Fixes #45.

This PR makes `Rails/Delegate` aware of `self`.

Author: koic

changelog/new_make_rails_delegate_aware_of_self.md

@@ -0,0 +1 @@
+* [#45](https://github.com/rubocop/rubocop-rails/issues/45): Make `Rails/Delegate` aware of `self`. ([@koic][])

lib/rubocop/cop/rails/delegate.rb

@@ -24,6 +24,14 @@ module Rails
       #   # good
       #   delegate :bar, to: :foo
       #
+      #   # bad
+      #   def bar
+      #     self.bar
+      #   end
+      #
+      #   # good
+      #   delegate :bar, to: :self
+      #
       #   # good
       #   def bar
       #     foo&.bar
@@ -60,7 +68,7 @@ class Delegate < Base
 
         def_node_matcher :delegate?, <<~PATTERN
           (def _method_name _args
-            (send (send nil? _) _ ...))
+            (send {(send nil? _) (self)} _ ...))
         PATTERN
 
         def on_def(node)
@@ -74,7 +82,11 @@ def on_def(node)
 
         def register_offense(node)
           add_offense(node.loc.keyword) do |corrector|
-            delegation = ["delegate :#{node.body.method_name}", "to: :#{node.body.receiver.method_name}"]
+            body = node.body
+
+            receiver = body.receiver.self_type? ? 'self' : ":#{body.receiver.method_name}"
+
+            delegation = ["delegate :#{body.method_name}", "to: #{receiver}"]
             delegation << ['prefix: true'] if node.method?(prefixed_method_name(node.body))
 
             corrector.replace(node, delegation.join(', '))
@@ -106,6 +118,8 @@ def include_prefix_case?
         end
 
         def prefixed_method_name(body)
+          return '' if body.receiver.self_type?
+
           [body.receiver.method_name, body.method_name].join('_').to_sym
         end
 

spec/rubocop/cop/rails/delegate_spec.rb

@@ -2,10 +2,6 @@
 
 RSpec.describe RuboCop::Cop::Rails::Delegate, :config do
   let(:cop_config) { { 'EnforceForPrefixed' => true } }
-  let(:config) do
-    merged = RuboCop::ConfigLoader.default_configuration['Rails/Delegate'].merge(cop_config)
-    RuboCop::Config.new('Rails/Delegate' => merged)
-  end
 
   it 'finds trivial delegate' do
     expect_offense(<<~RUBY)
@@ -33,6 +29,19 @@ def foo(baz)
     RUBY
   end
 
+  it 'finds trivial delegate to `self`' do
+    expect_offense(<<~RUBY)
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        self.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      delegate :foo, to: self
+    RUBY
+  end
+
   it 'finds trivial delegate with prefix' do
     expect_offense(<<~RUBY)
       def bar_foo
@@ -46,6 +55,19 @@ def bar_foo
     RUBY
   end
 
+  it 'finds trivial delegate to `self` when underscored method' do
+    expect_offense(<<~RUBY)
+      def bar_foo
+      ^^^ Use `delegate` to define delegations.
+        self.bar_foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      delegate :bar_foo, to: self
+    RUBY
+  end
+
   it 'ignores class methods' do
     expect_no_offenses(<<~RUBY)
       def self.fox